A common type of radiometer for detecting the presence of specific materials is provided with separate mixers and local oscillators for each frequency used in the measurement process. An example of this design is found in Guiraud, F.O. et al. "A Dual-Channel Microwave Radiometer for Measurement of Precipitable Water Vapor and Liquid", IEEE Trans. Geosci. Electron, GE-17 (1979), pp. 129-136. This design produces a stable, high-sensitivity instrument, but the high cost of millimeter wave components limits the implementation of a network of such devices.
As an alternative to this design, shared mixer radiometers are successfully used in networks of radiometric profiling systems since their cost is much lower than that of the design disclosed in Guiraud et al. However, shared mixer designs require a switchable frequency local oscillator having sufficient stability so as not to degrade the radiometer's accuracy even when the local oscillator is repeatably switched.
Accurate radiometer measurement requires a high number of readings to be repeatably taken at different frequencies. Repeatability, that is, the capability to maintain the same frequency characteristic through multiple frequency switching processes, is a critical factor in local oscillator design for the shared mixer configuration.
One type of local oscillator for radiometer use consists of multiple Gunn oscillators connected to a power combiner. This arrangement allows any frequency to be selected by applying power to one Gunn oscillator at a time. The major drawback of this technique is lack of temperature stability of the Gunn oscillators when they are turned on and off. To compensate, long settling times are required between frequency changes, thus reducing the efficiency and sensitivity of the radiometer.
One solution to the aforementioned problem is the use of PIN diode switches to improve temperature stability. The PIN diode switches select one Gunn oscillator at a time so that power is always applied to all the oscillators. This method is more completely described in the publication Schroeder, J.A. et al., "Design considerations of a network of thermodynamic profilers", J. Atmos. Oceanic Technol. (1989). While this technique decreases the time between frequency changes, it also increases the cost of the local oscillating system.
Another type of stable, switchable local oscillator requires use of a phase lock loop. This technique produces the most stable and repeatable source possible. However, a separate stable frequency source and other millimeter wave components such as a second mixer and coupler are required. Consequently, the cost of a phase lock loop oscillator system is too great to practically be used throughout an entire network of radiometer profilers.
Another way to achieve an inexpensive and stable local oscillating system for a shared mixer radiometer is the use of frequency multiplied oscillators. A stable low-frequency oscillator is used to provide a signal which is then multiplied up to millimeter wave frequencies. However, drift and stability of the low frequency oscillator are also multiplied along with the frequency. Consequently, stringent requirements are placed on the low frequency source. The costs of the frequency multipliers and a high quality low frequency source can exceed the costs of other local oscillator designs.
An important concern in local oscillator design for use in shared mixer radiometers is a balance between two factors. The first is oscillator stability required for radiometer use. The second is oscillator cost which mitigates against the deployment of large numbers of local oscillators in a multiple radiometer network.